The Doctor treats a Soviet soldier who has been exposed to something in the sea and persuades the Soviet Commander, Soren, not to suicidally attack the base. Returning to the church they find a new inscription on the wall before they in turn are found by Millington who shows them a chamber when the Navy are collecting a poison from a source inside. The poison is being gathered as a weapon to end the war and some has been secreted in the core of Judson's Ultima machine, which programmed to detonate it when it receives the word love. Whitehall wishes the Soviets to steal the core so they may be dealt with after the war. Two girls evacuated to the village swim in the sea and are consumed by a mist transforming them into vampire like creatures which attack and kill first a Soviet soldier, then their landlady. They come for the Reverend Wainright but he is saved by the Doctor & Ace. Ace reveals to the Doctor that she has unwittingly told Judson that the instructions are a logic diagram. He has fed it into the Ultima machine which has gone into overdrive. The Doctor is unable to stop it as mutated vampire figures rise from the sea bed.

On one level this seems quite simple: UK Forces have found a poison and are using it as weapon/trap. But into this we've got lots of Norse references, something bad having happened a long time ago, creatures rising from the sea and a seemingly inexplicable confiscation of all chess sets on the base!

When location scouting was being carried out for this story it was realised that it was possible to shoot the entire four part story on location, a distinction shared with Spearhead from Space & (sort of) Greatest Show in the Galaxy. The Naval base scenes were filmed at Crowborough Training Camp in East Sussex, while most of the rest of the locations are just across the border into Kent filmed in a town called Hawkhurst. St Lawrence's Church substitutes as St. Jude's Church where Nicholas Parsons claims he was mistaken for a real vicar! Lillesden School provides Millington's secret lab and Roses Farm is the cottage where the two girls have been evacuated to.

The only location not found in the vicinity is Lulworth Cove in Dorset which provides the beach and sea locations used during the story. This location was chosen due to the director having worked their before and two members of the production team having experience diving in the location. Both the preceding story, Ghost Light, and the following one, Survival, use locations nearby.